1. Field of the Invention
The present invention relates generally to an optical information storage apparatus for recording and reading information on and from an information storage medium through an optical head, and more specifically to an optical information storage apparatus for a write-once read-many optical disc type.
2. Description of the Prior Art
Apparatus for recording and reading information on and from an information storage medium through optical head means (e.g. a laser beam) are well known. As one of the information storage media, an information storage medium of write-once read many (WORM) optical disc type is also known. Since the storage medium of WORM type is not erasable, information are recorded in sequence on areas where information data have not yet been recorded. Further, in the WORM optical disc, information data are recorded by forming pits with different pit lengths and pit pitches in spiral fashion on the disc, and read by applying a laser beam to the pits to detect recorded data on the basis of physical change in the laser beam, in the same way as in information storage media of read-only type (called compact disc).
The above-mentioned WORM type optical disc or an optical information storage medium has widely been used, because a great number of information data can be recorded and read in spite of a relatively simple system configuration. Further, since the volume of information data recorded on the optical disc is huge, the WORM type optical disc is usually divided into two areas. The first area is called a retrieval index storing area on which only retrieval indices representative of information documents, for instance are recorded for providing an easy retrieval of huge information recorded. The second area is called an information storing area on which various information are recorded being arranged on the basis of recorded retrieval indices.
In the above-mentioned optical disc, when an information document is required to be deleted, conventionally, the document is usually deleted indirectly only on retrieval index storing area by recording a delete mark (flag) thereon. In other words, when an information document is required to read through the apparatus, a retrieval index recorded on the retrieval index storing area is first read in sequence to check block numbers on which a target information document is recorded. In this step, if the retrieval index has already been deleted and therefore a delete flag rises, the information document corresponding to the deleted retrieval index is not read, indicating that the information document is deleted. That is, the information document is processed as if it were deleted from the disc on the basis of a delete mark recorded on the retrieval index storing area.
In the above-mentioned information storage apparatus however, although a retrieval index of an information document required to be deleted is deleted, since actual information data corresponding to the deleted retrieval index are still kept as they are, there exists a problem in that the deleted information document can be read by disregarding the delete mark recorded on the retrieval index storing area.
In other words, when a recorded information document is strictly secret and therefore required to delete perfectly and completely, the prior-art optical information storage apparatus is not satisfactory from the functional standpoint.